(d) Conversion into dextrin and dextrose. To some starch solution in a flask add a few drops of 25-per-cent. sulphuric acid, and boil for 15 minutes. Take some of the liquid, which is now clear, and show the presence of dextrin and dextrose.

9. Dextrin. -Add iodine solution to solution of dextrin; a reddish-brown colour is produced. The colour disappears on heating and reappears on cooling.

10. Glycogen.-Solution of glycogen is given round: (a) it is opalescent like that of starch.

(b) With iodine solution it gives a brown colour very like that given by dextrin. The colour disappears on heating and reappears on cooling. (c) By boiling with 25-per-cent. sulphuric acid for 15 to 20 minutes it is converted into grape sugar.

The carbohydrates are found chiefly in vegetable tissues, and many of them form important foods. Some carbohydrates are, however, found in or formed by the animal organism. The most important of these are glycogen, or animal starch; dextrose; and lactose, or milk sugar.

The carbohydrates may be conveniently defined as compounds of carbon, hydrogen, and oxygen, the two last named elements being in the proportion in which they occur in water. But this definition is only a rough one, and if pushed too far would include many substances, like acetic acid, lactic acid, and inosite, which are not carbohydrates. Research has shown that the chemical constitution of the simplest carbohydrates is that of an aldehyde, or a ketone, and that the more complex carbohydrates are condensation products of the simple ones. In order, therefore, that we may understand the constitution of these substances, it is first necessary that we should understand what is meant by the terms aldehyde and ketone.

A primary alcohol is one in which the hydroxyl (OH) is attached to the last carbon atom of the chain; its end group is CH2OH. Thus the formula for common alcohol (primary ethyl alcohol) is

CH3.CH2OH.

The formula for the next alcohol of the same series (primary propyl alcohol) is

CH3.CH.CH2OH.

If a primary alcohol is oxidised, the first oxidation product is called an aldehyde; thus ethyl alcohol yields acetic aldehyde :

CH..CH,OH+O=CH.CHO+H,O.

[ethyl alcohol]

[acetic aldehyde]

The typical end-group CHO of the aldehyde is not stable, but is easily oxidisable to form the group COOH, and the compound so formed is called an acid; in this way acetic aldehyde forms acetic acid:

The majority of the simple sugars are aldehydes of more complex alcohols than this: they are spoken of as aldoses. The readiness with which aldehydes are oxidisable renders them powerful reducing agents, and this furnishes us with some of the tests for the sugars.

Let us now turn to the case of the ketones. A secondary alcohol is one in which the OH group is attached to a central carbon atom; thus secondary propyl alcohol has the formula

CH3.CHOH.CH,.

Its typical group is therefore CHOH. When this is oxidised, the first oxidation product is called a ketone, thus :—

CH,.CHOH.CH+O=CH.CO.CH,+H,O.

It therefore contains the group CO in the middle of the chain. Some of the sugars are ketones of more complex alcohols: these are called ketoses. The only one of these which is of physiological interest is levulose.

6

The alcohols of which we have already spoken are called monatomic, because they contain only one OH group. Those which contain two OH groups (like glycol) are called diatomic; those which contain three OH groups (like glycerin) are called triatomic; and so on. The hexatomic alcohols are those which contain six OH groups. Three of these hexatomic alcohols with the formula C,H,(OH)。 are of physiological interest; they are isomerides, and their names are sorbite, mannite, and dulcite. By careful oxidation their aldehydes and ketones can be obtained; these are the simple sugars; thus, dextrose is the aldehyde of sorbite; mannose is the aldehyde of mannite; levulose is the ketone of mannite; and galactose is the aldehyde of dulcite. The sugars all have the empirical formula C6H12O6. The constitutional formula for dextrose is :

By further oxidation, the sugars yield acids with various names. If we take such a sugar as a typical specimen, we see that their general formula is

C1H2mOm

and as a general rule n=m; that is, the number of oxygen and carbon atoms is equal. This number in the case of the sugars already mentioned is six. Hence they are called hexoses.

Sugars are known to chemists, in which this number is 3, 4, 5, 7, &c., and these are called trioses, tetroses, pentoses, heptoses, &c. The majority of these have no physiological interest. It should, however, be mentioned that a pentose has been obtained from the nucleoprotein of the pancreas, of the liver, and of yeast. If the pentoses that are found in various plants are given to an animal, they are excreted in great measure unchanged in the urine.

The hexoses are of great physiological importance. The principal ones are dextrose, levulose, and galactose. These are called monosaccharides.

Another important group of sugars is that of the disaccharides: these are formed by the combination of two molecules of monosaccharide together with the loss of a molecule of water, thus:

C6H12O6+C6H12O6=C12H22O11+H2O.

The principal members of the disaccharide group are cane sugar, lactose, and maltose.

If more than two molecules of the monosaccharide group undergo a corresponding condensation, we get what are called polysaccharides. nCH2O=(CHO5)n+nH,O.

The polysaccharides are starch, glycogen, various dextrins, cellulose, gums, &c. We may therefore arrange the important carbohydrates of the hexose family in a tabular form as follows:

3. Polysaccharides or Amyloses (0,H1005)

+ Starch.

+ Glycogen.
+ Dextrin.
Cellulose.

The signs and in the above list indicate that the substances to which they are prefixed are dextro- and levo-rotatory respectively as regards polarised light. The formulæ given above are merely empirical; the quantity n in the starch group is variable and often large. The following are the chief facts in relation to each of the principal carbohydrates.

For a description of polarised light and polarimeters see Appendix. This and the other matter in the Appendix are placed there for convenience, not because they are unimportant. Students are therefore urged to refer to and carefully study these subjects.

MONOSACCHARIDES

Dextrose or Grape Sugar.-This carbohydrate is found in fruits, honey, and in minute quantities in the blood (0-12 per cent.) and numerous tissues, organs, and fluids of the

body. It is the form of sugar found in large quantities in the blood and urine in the disease known as diabetes.

FIG. 1.-Dextrose crystals.

Dextrose is soluble in hot and cold water and in alcohol. It is crystalline (see fig. 1), but not so sweet as cane sugar. When heated with strong potash certain complex acids are formed which have a yellow or brown colour. This constitutes Moore's test for sugar. In alkaline solutions dextrose reduces salts of silver, bismuth, mercury, and copper. The reduction of cupric hydrate to cuprous hydrate or oxide constitutes Trommer's test, which has been already described at the head of the lesson. On boiling it with an alkaline solution of picric acid, a dark red opaque solution due to reduction of the picric to picramic acid is produced. Another important property of grape sugar is that under the influence of yeast it is converted into alcohol and carbonic acid (C6H12O6=2C2H6O+2CO2).

Dextrose may be estimated by the fermentation test, by the polarimeter, and by the use of Fehling's solution. The last method is the most important: it rests on the same principles as Trommer's test, and we shall study it and other methods of estimating sugar in connection with diabetic urine (see Lesson XII.).

Levulose. When cane sugar is treated with dilute mineral acids it undergoes a process known as inversion-i.e. it takes up water and is converted into equal parts of dextrose and levulose. The previously dextro-rotatory solution of cane sugar then becomes levorotatory, the levo-rotatory power of the levulose being greater than the dextro-rotatory power of the dextrose formed. Hence the term inversion. The same hydrolytic change is produced by certain ferments, such as the invert ferment of the intestinal juice, and of yeast.

Pure levulose can be crystallised, but so great is the difficulty of obtaining crystals of it that one of its names was 'uncrystallisable sugar.' Small quantities of levulose have been found in blood, urine, and muscle. It has been recommended as an article of diet in diabetes in place of ordinary sugar; in this disease it does not

C

appear to have the harmful effect that other sugars produce. Levulose gives the same general reactions as dextrose.

Galactose is formed by the action of dilute mineral acids or

*

FIG. 2.-Inosite crystals.

inverting ferments on lactose or milk sugar. It resembles dextrose in being dextro-rotatory, in reducing cupric hydrate in Trommer's test, and in being directly fermentable with yeast. When oxidised by means of nitric acid it, however, yields an acid called mucic acid (CH1008), which is only sparingly soluble in water. Dextrose when treated in this way yields an isomeric acid-i.e. an acid with the same empirical formula, called saccharic acid, which is readily soluble in water.

Inosite, formerly called muscle sugar, is found in muscle, kidney, liver, and other parts of the body in small quantities. It is also largely found in the vegetable kingdom. It is a crystallisable substance (see fig. 2) and has the same formula (C,H12O6) as the glucoses. It is, however, not a sugar. It gives none of the sugar tests, and careful analysis has shown it has quite a different chemical constitution from the true sugars. It belongs to the aromatic series, and is only included here for convenience.

DISACCHARIDES

Cane Sugar. This sugar is generally distributed throughout the vegetable kingdom in the juices of plants and fruits, especially the sugar cane, beetroot, mallow, and sugar maple. It is a substance of great importance as a food. After abundant ingestion of cane sugar traces may appear in the urine, but the greater part undergoes inversion in the alimentary canal.

Pure cane sugar is crystalline and dextro-rotatory. It holds cupric hydrate in solution in an alkaline liquid—that is, with Trommer's test it gives a blue solution. But no reduction occurs on boiling. After inversion it is strongly reducing.

Inversion may be brought about readily by boiling with dilute mineral acids, or by means of an inverting ferment, such as that occurring in the succus entericus or intestinal juice. It then takes up water and is split into equal parts of dextrose and levulose: